Amplifier with oscillation producing adjustment



June 21, 1938. M. S KLINEDINST ET AL 2,121,434

AMPLIFIER WITH OSCILLATION PRODUCING ADJUSTMENT Filed March 18, 1936Microphone flmplifier ana 2 47 Calibraithg Signal Generator Stage V 43INVENTORS Milford dilflinedinsi Olin B. Gunby Herbert Belar PatentedJune 21, 1938' AMPLIFIER WITH OSCILLATION PRODUC ING ADJUSTMENT MilfordS. Klinedinst, Philadelphia, Pa., Olin B.

Gunby, Merchantville, N. J., and Herbert Belar, 1 Philadelphia, Pa.,assignors to Radio Corporation of America, a corporation of DelawareApplication March 18, 1936, Serial No. 69,468

7 Claims.

The present invention relates to amplifiers and more particularly toamplifiers of the electric discharge type comprising a series of cascadeconnected amplifiertubes for sound recording and mg; the like whichrequire calibration or adjustment prior to use.

In the operation of such amplifiers it is desirableto apply thereto asound signal or audio frequency oscillations from a suitable source andthen to adjust the amplifier characteristic such as the ground noiselevel, for example, in a recording amplifier, whereby it is in conditionfor operation.

It is an object of the present invention to provide an audio frequencyamplifier having an amplifier stage which may be adjusted to produce acalibrating signal or oscillations, thereby eliminating additionalsignal sources heretofore provided for calibration purposes.

It is also an object of the present invention to provide an audiofrequency amplifier having an amplifier stage, the input and outputcircuits of which may be intercoupled to produce oscillations, andhaving means for simultaneously increasing the anode potential anddecreasing the output signal potential, whereby the converted stage isadapted to produce calibrating oscillations in the amplifier system ofwhich the stage forms a part.

In accordance with the invention, the amplifier and calibrating signalgenerator stage may be provided in an amplifier system preceding thatportion of the amplifier system in which the calibrating adjustments aremade in the presence of a fixed predetermined signal. amplifier andsignal generator stage is preferably located in the input stage of theamplifier or adjacent thereto.

By providing an amplifier stage and oscillator, the amplifiercharacteristic may be measured at a frequency or a plurality ofdifferent frequencies, depending upon the range of operation of theoscillator stage, which is preferably tunable over a pre-determinedsignal range. In order that a and in order to prevent overloading theoutput stages of the amplifier.

Accordingly, it is an object of the invention to Accordingly, the

minimum of apparatus may be used and in order provide an improvedswitching system for the output circuit of an audio frequency amplifierwhereby the same may provide normal impedance coupling and apre-determined potentiometer output connection when the input and outputcircuits thereof are coupled for the production of oscillations.

The invention will, however, be better understood from the followingdescription when considered in connection with theaccompanying drawingand its scope will be pointed out in the appended claims. I

In the drawing, Fig. 1 is a schematic circuit diagram of an audiofrequency amplifier adapted for recording sound signals and embodyingthe invention, and

Fig. 2 is a similar schematic circuit diagram source of signals 5, isconnected to the input trans former 6 of an audio frequency amplifierhaving a first stage electric discharge amplifier device i, the grid 8of whichis connected to the secondary 9 of the input transformer and theanode Ill-of which is connected through an output, coupling resistor IIand a source of anode potential l2. The anode and grid circuits arecompleted to the cathode 13 through ground connections M, l5 and IS, theground connection I5 including a bias, source of potential I! for thegrid 8, and the ground connection i4 being connected to the negativeterminal of the anode supply source I2.

Both the grid and the anode circuits are proplifier device from itsnormal function ,in the amplifier stage to an oscillator for producingoscillations at a fixed pre-determined frequency or variable range offrequencies within the operating range of the amplifier.

In the present example the grid circuit indicated at 26] and includingthe secondary 9 of the input transformer isconnected with the biassource I! through the grid circuit switching means comprising a switcharm 21 movable be- 7 tween a contact 22 and a contact 23, the contact 22being connected with the bias source I! through a negative bias supplylead indicated at 24. I

When the arm 2| of the switch is moved at the alternate connection 23,the grid circuit is arranged to'include the secondary 25 of anoscillatio-n coupling transformer 26 and the cathode connection iscompleted through a lead indicated at 21. The secondary 25 is tunableby' a shunt vided with'switching' means for changing the am 5' capacitor28, which may be variable, as indicated, to provide a relatively widerfrequency range of adjustment for the tuned circuit, thereby formed withthe secondary 25. A grid leak and condenser 29 and 30, respectively, areprovided in the grid circuit in connection with the transformer 26 tosupply bias to the grids 8 when operating as an oscillator.

When the switch isset for the production of oscillations and theconnection between 2| and 23 is completed, the grid circuit may betraced from the grid 8 through the grid circuit 20, including thesecondary 9, the switch arm and contact 2| and 23, respectively, thegrid leak and condenser combination 29 and 30, the tuned grid circuit25-48 and the cathode return lead 21 to the cathode l3. The latter maybe heated by any suitable means, such as a battery indicated at 3|.

The anode circuit switching means comprises a similar switch arm 32 andcontacts 33 and 34, the former providing connection for the anodecircuit 35 from the anode ID to the anode end 36 of the couplingresistor ll. At the same time, the contacts 32 and 33 serve to shortcircuit a series resistor 31 arranged to be inserted in series betweenthe anode and coupling resistor I I to provide a potentiometer outputconnection for the anode circuit, the output tap point of which isindicated at 38 and which is coupled to the sue-- ceeding stage throughany suitable means, such as a coupling capacitor 39 and the usual gridcircult resistor 40. In the present example the resistor 40 is providedwith a variable tap connection 4| for the grid 42 of the succeedingamplifier device 43, the grid circuit of which is completed throughconnection with the bias supply lead 24 and the ground I5, the cathode44 being connected in parallel with the cathode l3 by leads not shownfor the purpose of simplifying the drawing.

The amplifier device 43 is provided with an output anode 45, which iscoupled to an output coupling resistor 46 and coupling capacitor 41 witha coupling transformer 48 for the remainder of the amplifier indicatedin the rectangle 49, a plurality of amplifier stages being indicated bythe device 50. The output connection for the amplifier 43 is indicatedby the lead which is connected with the primary 52 of the transformer 48and is completed to the cathode through connection with the ground l4. 7

The amplifier 49 is provided with adjustment means indicated at 53,which may be any means for changing the operatingor frequencycharacteristic thereof such, for example, as means to reduce the groundnoise level or other calibrating means for the operation of which acalibrating signal is required to be applied to the amplifier. Arecording or other suitable output device is connected to the amplifierand is indicated at 54.

It will be noted that the anode circuit for the device 43 for the supplyof direct current potentials is connected through a lead indicated at 55with the source of anode potential l2 in common with the anode circuit35 for the device 1'. The alternate anode circuit connection for thedevice 1 provided by the switch connection 32--34 is completed also tothe potential source l2 through the primary 56 of the transformer 26 andserves to provide a feed-back connection from the anode ID to the grid8, the anode current being permitted to flow through the primaryWinding, which has a relatively low resistance; to anode current flow ascompared with the coupling resistor ll.

Accordingly, the anode potential on the combined amplifier andoscillator device is materially increased for the production ofoscillations, while at the same time, by the introduction of the seriesresistor 31, the output signal potential is reduced as applied to thesucceeding stage or volume control device 40-4l, whereby the latter maybe adjusted to prevent over-loading the succeeding amplifier stageswithin the limits of its control range.

The switch means is preferably of unitary construction providing aconnection between the movable arm 32 and the movable arm 2 l, asindicated by the dotted connection 51. With this arrangement, the gridand plate circuits are simultaneously changed from amplifier operationin the positions shown to the alternate oscillator position, providing acoupling to the transformer 25. When the oscillator is providingoscillations the same may be adjusted in frequency by adjustment of thetuned oscillator secondary circuit, and in the present example, byadju'stmen of the capacitor 28.

It' will be seen that the oscillator connection provides that theresistors 31 and II are connected in series across the primary winding56 and that the output connection 38 is in effect a tap on anintermediate point of the potentiometer, thus formed, while foramplifier operation the full voltage output is taken from the resistor lI alone through the coupling capacitor 39, thereby rendering theamplifier normally efiicient for the transmission of signals but for theproduction of oscillation, causing a material reduc tion in the output,while at the same time providing strong oscillations through theoperation of increasing the anode potential, as hereinbefore described.

When adjusted for amplifier operation signals picked up by themicrophone are suitably amplified and applied to the recording or outputdevice 54, while for calibration operationthe amplifier stage includingthe device I is adjusted for the production of pro-determinedoscillations within the amplifier range and the same are applied to theamplifier which may then be adjusted in the presence of such signals formost effective operation through any-adjusting means, such as theelement 53 indicated, in the drawing.

Referring now to Fig. 2, an amplifier stage just preceding a poweramplifier is shown with the provision for generating oscillationswhereby the amplifier characteristic may be determined at any time.

In the present figure a power amplifier is indicated at 6!] and isprovided with a driver or pre-amplifier stage, including an electricdischarge amplifier device Gl. The device 6 is provided with a controlgrid 62 coupled to an input circuit 83 through a coupling transformer 54and is arranged to receive biasing potential from a fixed source65 orfrom a grid leak and condenser combination 65 through a selector switchbe changed to provide a potentiometer output andto the output volumecontrol device 16,-, The volume control device is coupled to the poweramplifier through a suitable coupling transformer TI and the circuit isgrounded as indicated at 18. Ihe switches 61 and 69 are interconnectedfor simultaneous operation by the mechanical connection indicated at 18.

In this system the reduced signal output for the oscillator is providedby the potentiometer device 116, the contact 19 of which is connected tothe high potential end 80 thereof by switch means 8| operable with theswitches 61 and 69 when they are closed to permit normal amplifieroperation, thereby restoring the full output from that established byadjustment of the contact 19 when calibrating the amplifier 60.

This arrangement is desirable when, from time .to time, it is necessaryto test an amplifier without including a pre-amplifier and permits theuse of the combined amplifier and calibrating signal generating stage atan intermediate point in an amplifier system and, by reason of theadjustment of the frequency range of the oscillater, the output responsecharacteristic of the amplifier to be tested may be determined over arelatively wide range of frequencies, depending upon the frequency rangeto be covered by the tuned circuit 68.

The present circuit has the further advantage that the signal volume oroutput level forthe oscillator may be adjusted independently of theoutput level for amplifier operatiomthe full output being taken for thelatter, while at the same time the anode potential is increased toinsure operation of the device as an oscillator with increased anodepotential, since the resistance of the primary I2 is normally lower thanthe resistance of the coupling resistor 14. r

In any case, it is desirable that the input and output circuits besimultaneously adjusted by common means and that the output connectionconnection when generating oscillations and preferably with adjustablecontrol as in the circuit of Fig. 2. Furthermore, the anode circuitswitching means provides for the feed-back connection through a lowresistance coupling device. across which the output potentiometer meansis connected. e V

In the grid circuit it will be noted that the high potential side of thecircuit is not changed and that the switching is provided in the lowpotential side thereof, thereby to have a minimum of high frequencylosses in a high fidelity circuit as may be used for recording purposes.

We claim as our invention:

1. In a sound recording electric discharge amplifier, an amplifier stageincluding an electric discharge amplifier device, switching means forcouplingthe input and output circuits of said stage to produceoscillations, tuning means therefor for causing said oscillations tofall within the frequency range of operation of said ampliiier and meansfor reducing the amplitude of said oscillations to a pre-determinedlevel in response to the actuation of said switching means, and meansfor impressing said reduced amplitude oscillations upon the remainder ofsaid amplifier.

2, In an audio frequency amplifying system, means for establishing acalibrating signaltherefor having a pre-determined frequency rangewithin the frequency range of operation of said amplifier, said meanscomprising an amplifier last-narned circuit to be selectively connectedto said output circuit, and means in said output circuit for reducingthe amplitude of oscillations derived from said amplifier and controlledpre-amplifier portion, tunable means for coupling the input and outputcircuits of said amplifier stage to produce oscillations in said stage,means for conducting said oscillations therefrom at a reduced amplitudelevel and switching means for controlling said coupling and last namedmeans.

4. The combination with an audio frequency amplifier stage having aninput circuit and an output circuit, of means providing a tunablefeedback coupling connection between said input circuit and said outputcircuit, means for conjointly rendering said last named means effectiveand reducing the signal output from said stage, said last named meanscomprising an anode coupling resistor, a second resistor in seriestherewith at the anode end thereof, a signal output connection at theanode end of said first named resistor, and switching means forconnecting said output circuit to said first-named means and insertingsaid second named resistor in series with the first named resistor,thereby to increase the anode potential on said device and to reduce thesignal output therefrom.

5. In an audio frequency amplifier, an amplifier stage comprising anelectric discharge device having grid, cathode and anode electrodes,signal input and bias potential supply means connected between said gridand cathode, an output circuit including anode potential supply meansconnected with the anode, an output coupling resistor, a tunablefeed-back coupling device, means for connecting said resistor andcoupling device selectively into said output circuit, said couplingdevice having a lower resistance than that of the coupling resistor, andmeans providing a selective circuit connection for said grid throughsaid input circuit with said coupling device to the exclusion of saidbias supply means, whereby said grid and anode are coupled for theproduction of oscillations.

6. A self calibrating electric discharge audio frequency amplifiercomprising, in combination, an amplifier portion having means forcontrolling the operation thereof whereby said portion may becalibrated, and means in said amplifier pro- ,viding a precedingamplifier stage having a confrequency, and means for reducing theoutput.

therefrom to a pre-determined adjusted value in response to theadjustment for oscillator operation.

'7. A self calibrating electric discharge audio frequency amplifiercomprising, in combination, an amplifier portion having means forcontrol ling the operation thereof whereby said portion may becalibrated, and means in said amplifier providing a preceding amplifierstage having a connection for operation as an audio frequencyoscillator, means for tuning said last named stage to produceoscillations of a predetermined audio frequency, means for reducing theoutput therefrom to a predetermined adjusted value in reback couplingcircuit, means" for causing said' bysaid last-named means to beconnected efiecsponse to the adjustment for oscillator operat'ion', saidlast named means including a, resistorcapacitor impedance couplingnetwork for the output of said stage and a feed-back couplingtransformer having a primary winding, and.

switching means for connecting the output cir-

